Plasma deposition refers to any of a wide variety of processes in which a plasma is used to assist in the deposition of thin films or coatings onto the surfaces of objects. For example, plasma deposition processes are widely used in the electronics industry to fabricate integrated circuits and other electronic devices, as well as to fabricate the magnetic tapes and disks used in audio, video, and computer applications. Plasma deposition processes may also be used to apply coatings to various objects to improve or change the properties of the objects. For example, plasma deposition processes may be used to apply wear resistant coatings to machine tools, while other types of coatings may be used to increase the corrosion resistance of other items, such as bearings, turbine blades, etc., thereby enhancing their performance. In still other applications, plasma deposition may be used to apply coatings to various types of surfaces in the optics and glass industries.
In most plasma deposition processes the plasma is created by subjecting a low-pressure process gas (e.g., argon) contained within a vacuum chamber to an electric field. The electric field, which is typically created between two electrodes, ionizes the process gas, creating the plasma. If direct current (DC) is used to produce the electric field, the negatively charged electrode is usually referred to as the cathode, whereas the positively charged electrode is referred to as the anode. Thus, in the case of a DC sputter deposition plasma process, the material to be deposited on the object or substrate is usually connected as the cathode, whereas some other element, typically the vacuum chamber itself, is connected as the anode. Ionized process gas atoms comprising the plasma are accelerated toward the negatively charged cathode which also includes a target containing the material to be deposited on the substrate. The process gas atoms ultimately impact the target material and dislodge or sputter atoms from the target, whereupon the sputtered atoms subsequently condense on various items in the chamber, including the substrate that is to be coated. The substrate is usually positioned with respect to the target so that a majority of the sputtered target atoms condense onto the surface of the substrate.
While sputter deposition processes of the type described above may be used to deposit a wide variety of materials (e.g., metals and metal alloys) onto various substrates, they may be used to deposit compound materials as well. Reactive sputter deposition is the name usually given to sputtering processes which involve the sputtering of the target in the presence of a reactive species (e.g., oxygen or nitrogen gas) in order to deposit a film comprising the sputtered target material and the reactive species. A wide variety of compounds, such as SiO.sub.2, Al.sub.2 O.sub.3, Si.sub.3 N.sub.4, and TiO, can be deposited by reactive sputter deposition processes.
Regardless of the particular type of sputtering process being performed (e.g., non-reactive or reactive), the sputtering yield, i.e., the number of target atoms sputtered per incident ion, depends on the energies of the incident ions. The overall sputtering rate depends on both the energies of the incident ions as well as the total number of ions bombarding the target surface during a given time period. Therefore, in order to maximize sputtering efficiency, it is desirable to produce and confine the ions and electrons in the glow discharge as close as possible to the surface of the target material. Towards this end, numerous kinds of magnetically assisted sputtering cathodes or magnetrons have been developed which utilize magnetic fields to confine the glow discharge in a region close to the surface of the target being sputtered.
A typical planar magnetron may include a plate-like or planar target along with a magnet assembly suitable for producing a plasma-confining magnetic field adjacent the target. While numerous shapes and configurations of plasma-confining magnetic fields have been developed and used with varying degrees of success, it is common to shape the plasma-confining magnetic field so that it forms a closed loop ring or "racetrack" over the surface of the target material. When viewed in cross section, the flux lines of the magnetic field loop or arch over the surface of the target, forming a magnetic tunnel, which confines the glow discharge to the ring or racetrack shaped sputtering region. The shape of the predominate electron path defines the portion of the target material that will be sputtered.
Unfortunately, in most conventional magnetrons having such ring shaped or racetrack shaped magnetic tunnels, the arched shape of the magnetic field over the target surface tends to force or "pinch" the electrons, thus the predominate electron path, toward the center of the tunnel. This pinching effect causes the plasma density and, therefore, the sputtering erosion, to be highest along the centerline of the tunnel. As the target is gradually eroded, the pinching forces tend to strengthen, ultimately resulting in a V-shaped erosion groove in the target. The fraction of the target material that has been sputtered away by the time the bottom of the V-shaped erosion groove reaches the back surface of the target is referred to herein as the target utilization. In most magnetrons, the target utilization is relatively low, in the range of about 20% to 30%. Since the commonly used target materials tend to be relatively expensive, such low target utilization is wasteful and increases the costs associated with the sputtering process. For example, although spent targets may be recycled and reworked into new targets, the time spent changing and reworking targets can be significant and in any event, increases the overall cost of the sputtering operation. Therefore, any significant increase in target utilization translates directly into cost savings, as the increased target utilization enables longer production runs and less downtime spent in reworking and replacing targets.